1. Field of the Invention
The present invention relates to an image forming apparatus and the like including an automatic double-sided printing device, and, more particularly to an image forming apparatus including a mechanism for preventing a sheet from being heated by a fixing device to cause an image failure and a double-sided printing method for the image forming apparatus.
2. Description of the Related Art
Conventionally, as image forming apparatuses to which an electrophotographic technique is applied such as a facsimile and a laser printer, there is known an image forming apparatus of a circulation type that forms, in performing double-sided printing, an image on one side of a sheet in an image forming unit in an image forming apparatus main body and, after fixing the image with a fixing device, switches back the sheet with a paper discharge roller, conveys the sheet into an automatic double-sided printing device disposed on a side of the image forming apparatus main body, and reverses the sheet to feed the sheet to the image forming unit again. As such an image forming apparatus of the circulation type, there is known an image forming apparatus of a type that sequentially performs, after single-sided printing of a first sheet, single-sided printing of second and third sheets, simultaneously conveys plural sheets in an automatic double-sided printing device, and sequentially feeds the sheets from the automatic double-sided printing device to an image forming unit in order to improve productivity (see, for example, JP-A-2005-70223).
However, in the techniques described above, the image forming apparatuses have mechanisms in which a sheet is inevitably conveyed near the high-temperature fixing device again after single-sided printing of the sheet. Thus, for example, when processing by a CPU is delayed or supply of a heat quantity to the fixing device is delayed because of a large amount of image information, the sheet may be inevitably stopped near the fixing device during the switchback because of limitation in image processing. As time of this stop is longer, it is more likely that, in particular, a sheet for color printing is deteriorated when the sheet is left untouched in a high-temperature environment and color unevenness (image dust) occurs.
In other words, in double-sided printing in the circulation type performed by using a sheet exclusively used for color printing in the conventional color image forming apparatus, when a sheet being printed is inevitably stopped under the situation described above, an image failure occurs if a part of the sheet is located in a high-temperature portion near the fixing device.
On the other hand, image forming apparatuses in recent years tend to be improved in image quality, reduced in size, increased in speed, and used for color printing. Thus, double-sided printing of the circulation type is essential. Moreover, image processing is complicated and the length of a sheet conveying path is reduced.
Therefore, it is extremely difficult to provide a conveying path such that places where all sheets being conveyed are stopped are away from positions near high-temperature positions heated by the fixing device when a following sheet is temporarily stopped because of, for example, standby for processing of a preceding sheet.
As measures against such an image failure, a method of lowering a fixing temperature to control damage due to heat to a color printing sheet is conceivable. However, lowering of the fixing temperature makes development of toners and development of engine sections of image forming apparatuses inevitable. Therefore, expenses and personal expenses required for the development are enormous.